For me it'd be Ideological Purity on that which is most important to me, up to a certain point. After that point, it comes down to electability.
Hypothetically, if talking percentages, it'd be something along the lines of suggesting that as long as the guy is about 75% in line with what I think, then electability becomes a bigger concern.
More specifically, sometimes it's less about general ideology but specific issues AND track record. For example, if I care greatly about reforming entitlements, staunch border control/enforcement, legalization of marijuana, and 2nd amendment protection then someone who rates poorly on those four things to me is not going to get my vote simply because he's more "electable" than the other. This is either based on what the candidate has said on those things...OR what the candidate has shown himself quickly ready to compromise himself on in the past.
There are also more factors than just ideology and electability that go into my vote. But in a vacuum of just those two....the first hurdle is making it over a certain baseline needed to get my vote in terms of ideology, the second hurdle is to get over a certain hurdle in terms of electability, and as they come down the home stretch it becomes a balancing act of how far each candidate is beyond the minimum requirements for me comparitive to each other.
If you're someone whose going to make me groan more than cheer, whose going to be voting in favor of more damaging things in my mind than positive things, if you're someone who I just can't rightly feel correct in supporting...then I don't care if you're a 100% guarantee to win, you're not getting my vote. You're not getting my voice as a citizen stating "I support this person and what he chooses to vote for, because I'm giving him the ability to be my voice". If you will act and vote in a way that will leave me typically more happy than upset, but perhaps not AS happy as the other candidate would, but you're significantly more likely to win...Then yes, I'll cast a vote for you. But you have to get past that minimum threshold.